I have an Asus-D8 WS Z9PE which is equipped with two Xeon E5-2687W ES with the B0 stepping.
My problem is that the current Bios 0503, although both Xeons supported, but no later than GTX580 graphics cards.

If I install the latest BIOS 5701, I can operate my system stable with a Xeon, if I set the PCI-E support on GEN2.

The problem is that I can set the PCI-E support only when reset the bios, but since the 2nd Xeon is not enabled and I have to also adjust this setting to not have access to the GEN2.

I guess you'd have only one mod the default setting of BIOS 5701 to "Gen2" change of the PCI-E Supports "Auto" and then the system would run with two Xeons.